The demand on wireless data services has grown exponentially over the last ten years, driven particularly by the popularity of smart phones. To meet this growing demand, new generations of wireless standards with both multiple input and multiple output (MIMO) and orthogonal frequency division multiple access (OFDMA) and/or single carrier frequency division multiple access (SC-FDMA) technologies have been developed, such as 3GPP (3rd Generation Partnership Program) LTE (Long Term Evolution) and WIMAX (Worldwide Interoperability for Microwave Access). These standards prescribe base stations, such as eNBs in LTE, that divide geographic coverage areas into cells, with each base station serving at least one cell.
One challenge of capacity growth is the optimal use of the limited radio resources shared by multiple user equipment (UE), such as physical downlink control channel (PDCCH) use. In 3GPP LTE systems, the PDCCH carries important scheduling information and instructs the UE where to look for the physical downlink shared channel (PDSCH) and where to send the physical uplink shared channel (PUSCH). For voice over Internet protocol (VoIP) calls, the demand on PDCCH is high. Another challenge is maintaining performance of UEs at the cell edge.
The PDCCH carries downlink control information (DCI) which includes scheduling information for both the uplink and downlink. The DCI provides the UE with necessary information for proper reception and decoding of downlink data transmissions. A UE may be assigned one or more DCIs in a given transmission time interval (TTI). The PDCCH which carries the DCI consists of multiple control channel elements (CCE), where each CCE has multiple resource element groups (REG). Therefore, the REG is the building block of the PDDCH. Multiple REGs for each PDDCH for different users are interleaved and spread among multiple time-frequency resource elements (RE) in order to improve time and frequency diversity at the UE receiver for blind decoding. This allows for minimum inter-cell interference (ICI) among cells, where ICI arises from the signals transmitted into one cell carrying over into one or more neighboring cells.
As noted above, voice over Internet protocol (VoIP) calls place a high demand on the PDDCH. This is because the size of a VoIP packet is small and delay sensitive, therefore requiring a large control region. Thus, in the event of a high number of VoIP calls in a cell, the data region of the PDSCH may be wasted due to high use of the PDCCH. This is shown symbolically in FIG. 1. The data region of the PUSCH may also be wasted as a result of the high use of the PDCCH. In fact, for VoIP, PDCCH capacity is a key limiting factor as the demand on DCI is very high.
PDCCH link adaptation (LA) is used to choose an optimal CCE aggregation level for each DCI based on radio channel conditions, as measured and reported by the UE as a channel quality indicator (CQI). If the channel condition is good, i.e., for higher CQI, a fewer number of CCEs or a lower CCE aggregation level is used. Conversely, for lower CQI, a higher number of CCEs or a higher CCE aggregation level is used. Since the number of CCEs for each TTI is limited, the performance of PDCCH link adaptation will greatly impact the performance of the LTE radio access network. If the PDCCH LA is too aggressive, i.e., using fewer CCEs, some UEs will have a greater rate of PDCCH decoding failure, in which case, the UE cannot locate the related downlink data on the PDSCH or properly uplink data on the PUSCH. On the other hand, if PDCCH LA is too conservative, using greater numbers of CCEs, then fewer UEs can be accommodated by the available PDCCH resources, resulting in lower capacity. In addition to adjusting CCE aggregation level, PDCCH transmit power per user can be controlled to improve PDCCH detection at a UE.
Currently, there are only 4 CCE settings: 1, 2, 4 and 8. A problem with existing processes for adjusting a number of CCEs used for the PDCCH, is that CCE aggregation level setting is too coarse. Further, determination of the CCE aggregation level is currently too slow, resulting in less than optimal performance, especially for UEs at the cell edge. Also, power control of the PDCCH may result in higher interference for UEs in adjacent cells. PDCCH inter-cell interference coordination (ICIC) may be used to reduce the amount of interference experienced by a UE. However, the interference is still present. In case of a heterogeneous network (Hetnet) deployment of devices with different operating systems and protocols, interference can be eliminated, but only with a large waste of radio resources.